Wii: Not for a while (late next year at the earliest).
360: Early next year is a possibility, if the PS3's momentum continues, but I wouldn't be surprised if they waited til the business end of the year (as in, when things heat up in terms of sales), so that would be September 2010 (same time as Slim + Pricecut for ps3 this year).
PS3: This is a tough one. Assuming PS3 is break-even for Sony by March 2010 (isn't that what they have been indicating), the production cost could drop by another $30-50 by September 2010. If MS cut the 360 price by $50 at that stage (I can't see the Arcade going to 2010 at that stage at least, especially with Natal coming around the corner), it's possible we'll see Sony counter by cutting prices at exactly the same time. Or they could wait and bring in some profit from the PS3 for a change, especially if the 360 cut is small-ish ($50 as opposed to $100 cut).
All of this is pure predictions though, no rumours or news to back it up. It's all just what I'd guess.
Edit: And knowing me, I'll get it all wrong and we'll see all 3 systems hit $99 next Tuesday. 
Edit 2: Changed "last next year" in the Wii section to "late next year". It was originally a typo (last next year means absolutely nothing anyway).
